C    S    513    fllfl 


GIFT  OF 

Arthur  E»   Moncqster 


•  .  •;.••  s   -•  p*j  •. 


cular  WM509 


TheWestinghouse 


The   Application 

of 

J  aP 


ow   Pressure 
Turbines 


PITTSBUR.G,F»A. 


J-V 


i 


ler 


fl  5 


«  6 
6  6 

6  & 
•« 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 

BY 

FRANCIS     HODGKINSON          \    ^'iY/  ,jJj,>« 

ENGINEER  STEAM  TURBINE  DEPARTMENT,  THE  WESTINGH0LJSE  M 


Inasmuch  as  a  low  pressure  turbine  works  with  substantially  half 
the  heat  drop  of  a  high  pressure  condensing  turbine,  its  construction  is 
more  simple  and  there  are  fewer  turbine  elements.  As  the  volumes  of 
steam  which  pass  through  the  turbine  are  comparatively  greater,  large 
blade  passages  are  permitted,  all  of  which  conduces  to  the  simplicity  and 
ruggedness  of  the  turbine  and  facility  of  design.  In  low  pressure  turbine 
applications,  therefore,  the  skill  of  the  engineer  is  frequently  called 
upon,  more  in  connection  with  the  method  of  applying  the  turbine  than 
in  its  actual  design.  It  is  the  object  of  this  pamphlet  to  set  forth  some 
of  the  considerations  of  which  account  must  be  taken  in  making  such 
applications. 

The  following  may  be  cited  as  questions  which  arise  when  such  an 
installation  is  contemplated.  Is  there  enough  steam  for  the  low  pressure 
turbine,  and  has  its  quantity  been  properly  estimated?  Although  for 
preliminary  purposes  a  sufficiently  accurate  guess  of  the  available 
quantity  of  steam  may  be  made,  the  use  of  indicator  cards  from  the  high 
pressure  machines  is  advisable.  The  analysis  of  these  cards  may  be 
made  in  a  most  satisfactory  manner  by  the  method  described  before  the 
A.  S.  M.  E.  at  Cleveland  in  May,  1912,  by  Mr.  Paul  Clayton.  Steam 
meters  furnish  an  alternative  method  of  arriving  at  the  desired  figures, 
although  the  preferable  procedure  is  a  complete  test  of  the  boiler  plant 
itself.  It  is  advisable  to  make  a  deduction  of  10%  from  the  apparently 
available  amount  of  steam  to  allow  for  the  moisture  in  the  exhaust  from 
the  engines. 

A  second  typical  question  is  that  of  the  governing  of  the  turbine. 
Should  this  be  accomplished  by  the  direct  application  of  a  centrifugal 
device  to  the  turbine,  or  should  arrangements  be  made  to  tie  together 
electrically  the  generators  driven  by  the  turbine  and  by  the  reciprocator 
furnishing  the  exhaust  steam.  If  a  governor  is  used,  acting  directly  on 
the  turbine,  should  it  control  the  inlet  to  the  turbine  alone,  or  should  it  be 
connected  to  a  valve  in  the  low  pressure  line  permitting  a  portion  of  this 
steam  to  pass  directly  to  the  condenser  and  thus  lowering  the  back  pres- 
sure on  the  reciprocating  engines  whenever  possible,  due  to  low  load  on 
the  turbine?  Should  the  turbine  be  complicated  and  its  overall  effic- 
iency somewhat  reduced  by  the  addition  of  a  high  pressure  element  so 
that  it  will  operate  with  passable  efficiency  on  steam  direct  from  the 
boilers? 


726304 


4  THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 

Finally,  should  the  low  pressure  turbine  be  installed  at  all?  Does 
the  condition  of  the  reciprocating  engines  warrant  their  perpetuation,  or 
jyould  tlie^best' results  be  obtained  by  the  installation  of  complete  ex- 
pansion 'turbines?*'  T^is  condition  exists  in. many  plants  where  low  pres- 
"siHfe  • j&3t£llatioijs^a'Te  suggested. 

CLASSIFICATION. 

Low  pressure  turbine  installations  may  be  more  or  less  readily 
classified,  and  the  various  types  are  here  enumerated  for  convenience. 
A  more  detailed  discussion  of  each  appears  in  the  succeeding  pages. 

Case  A:  A  low  pressure  turbine  taking  steam  from  the  exhaust 
of  a  reciprocating  engine,  the  generators  of  each  being  connected  to  the 
same  bus  bars  and  no  governing  device  used. 

Case  B :  A  turbine  or  a  number  of  turbines  and  engines  connected 
similarly  to  Case  A. 

Case  C:  A  low  pressure  turbine  operating  in  conjunction  with  one 
or  more  engines  as  in  Cases  A  or  B,  except  that  the  turbine  and  engine- 
driven  generators  are  of  different  electrical  characteristics.  A  direct 
current  street  railway  generating  plant,  with  alternating  current  dis- 
tribution to  distant  substations,  is  a  good  example  of  this  case,  the 
turbine  and  engine-driven  generators  being  tied  together  by  rotary  con- 
verters or  motor  generators  sets.  Another  expedient  by  which  the  use 
of  a  governor  could  be  eliminated  is  the  connection  of  the  turbine-driven 
alternator  to  bus  bars  upon  which  floats  a  synchronous  motor  belted  or 
direct  connected  to  the  reciprocating  machine. 

Case  D :  A  low  pressure  turbine  operating  on  the  steam  exhausted 
by  a  number  of  engines,  pumps,  or  other  apparatus,  without  any  relation 
between  the  electrical  output  from  the  turbine  and  the  amount  of  steam 
available.  In  such  a  case  a  governor  controlling  the  admission  valve  of 
the  turbine  is  obviously  necessary,  as  is  a  relief  valve,  permitting  any 
excess  of  low  pressure  steam  to  pass  to  the  atmosphere. 

Case  E:  A  low  pressure  turbine  operating  on  the  exhaust  from 
engines  which  are  carrying  an  independent  load,  as  in  Case  D.  The 
turbine  governor,  however,  controls  a  valve  which  connects  the  recip- 
rocating engines  with  the  condenser,  imposing  on  them  only  enough 
back  pressure  to  enable  the  turbine  to  carry  its  load.  The  engines  thus 
have  the  benefit  of  some  vacuum  whenever  the  load  on  the  turbine  is 
light  enough  to  require  less  than  atmospheric  inlet  pressure. 

Case  F:  A  low  pressure  turbine  operating  in  conjunction  with  an 
engine  driving  a  mill  or  a  system  of  shafting,  the  output  of  the  turbine 
being  used  for  motors,  lights,  etc.,  and  any  excess  of  current  generated 
over  the  electrical  demand  may  be  returned  to  the  shafting  by  using  a 
synochronous  motor,  coupled  or  belted  to  the  line  shaft,  and  thus  acting 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES  5 

as  a  balance  to  proportion  the  load  between  the  two  machines  so  that 
the  best  economy  may  be  obtained. 

Case  G:  A  low  pressure  turbine  receiving  steam  from  an  inter- 
mittently operating  engine  such  as  a  hoisting  engine  or  a  rolling  mill 
drive.  If  the  interval  in  the  steam  supply  are  not  too  great,  a  regener- 
ator may  be  employed,  absorbing  the  excess  supply  of  steam  at  one  time 
to  give  it  up  again  to  the  turbine  when  the  latter  demands  an  amount 
exceeding  that  passing  from  the  engine. 

Case  H:  Practically  all  turbines  equipped  with  generators  have  a 
valve  which  will  admit  sufficient  live  steam  to  carry  the  normal  load 
should  the  low  pressure  supply  fail.  Such  an  arrangement  does  not, 
however,  give  high  efficiency  on  high  pressure  steam  since  its  expansive 
energy  is  wasted  in  throttling  and  only  a  small  amount  is  recovered  from 
the  resultant  superheat.  Case  H,  therefore,  provides  what  is  termed  a 
mixed  pressure  turbine  which,  in  addition  to  the  low  pressure  section,  is 
equipped  with  elements  enabling  it  to  expand  steam  from  boiler  pressure 
to  that  of  the  condenser.  Such  a  turbine  is,  of  course,  so  constructed 
that  all  the  available  low  pressure  steam  enters  it  at  the  proper  point. 
A  mixed  pressure  turbine  is,  therefore,  used  where  it  gives  better  overall 
efficiency,  although  it  has  a  poorer  economy  on  low  pressure  steam  alone 
due  to  the  dead  load  of  the  idle  high  pressure  element.  The  relative 
proportion  of  the  high  and  low  pressure  elements  will  be  determined  by 
the  amounts  of  steam  of  each  class  to  be  handled  and  the  continuity  with 
which  they  are  supplied.  Such  a  turbine  must,  of  course,  be  equipped 
with  a  governor. 

GENERAL. 

The  primary  general  consideration  in  any  low  pressure  installation 
is  to  insure  that  on  the  one  hand  the  turbine  will  receive  sufficient  steam 
to  enable  it  to  meet  all  demands  for  load;  and  on  the  other  hand,  to  in- 
sure that  all  the  available  exhaust  steam  is  utilized  either  in  the  turbine 
or  in  some  manner  even  more  efficient  about  the  plant.  The  installa- 
tion should,  therefore,  be  so  arranged  as  to  absolutely  preclude  the  pos- 
sibility of  exhaust  steam  escaping  to  the  atmosphere. 

In  this  connection  it  is  pertinent  to  state  that  while  the  low  pres- 
sure turbine  is  an  exceedingly  efficient  machine  mechanically  and  af- 
fords an  economical  means  of  using  a  by-product  often  wasted,  its 
absolute  thermal  efficiency  is  only  approximately  10%.  Exhaust  steam 
condensed  in  a  feed  water  heater,  on  the  other  hand,  shows  a  return  of 
approximately  80%,  so  that  such  a  heater  should  have  first  consideration 
in  any  low  pressure  installation.  In  general,  the  amount  of  exhaust 
steam  required  for  heating  the  feed  water  is  about  one-sixth  of  the  total 
amount.  In  several  cases  previously  cited,  the  pressure  in  the  receiver 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


from  which  the  turbine  draws  its  steam,  is  subject  to  variation.  It 
rises  with  the  load,  and  below  full-load,  is  apt  to  be  slightly  less  than 
atmospheric.  If  the  feed  water  heater  were  connected  with  this  receiver' 
the  temperature  would  be  low  when  the  receiver  pressure  was  low. 

In  such  a  case,  a  good  arrangement  would  be  to  take  exhaust  from 
all  the  plant  auxiliaries  and  collect  it  into  a  single  main  leading  to  the 
heater.  Then  any  steam  not  condensed  in  the  heater  would  pass  from 
it  through  a  constant  pressure  valve  such  as  shown  in  Fig.  1,  and  thence 
into  the  receiver  from  which  the  low  pressure  turbine  draws  its  steam. 
Thus  the  feed  water  heater  pressure  would  be  maintained  at  that  of  the 
atmosphere  and  the  water  passing  to  the  boiler  feed  pumps  would  be  at 
212°,  an  ideal  condition  and  one  readily  obtained  provided  the  amount 
of  exhaust  steam  from  the  auxiliaries  is  always  sufficient  to  produce  the 
desired  heating  effect.  Obviously,  the  heater  must  be  protected  by  a 
relief  valve  in  order  that  the  pressure  may  not  reach  an  unduly  high  value. 
This  valve  should  be  preferably  a  back  pressure  valve  of  first-class  make, 
absolutely  tight  and  capable  of  being  loaded  to  about  10  Ibs.  above  the 
maximum  receiver  pressure.  The  auxiliaries,  of  course,  must  be  capable 
of  operating  against  the  pressure  corresponding  to  the  highest  receiver 
pressure. 

It  is  advisable  in  all  low  pressure  turbine  work  that  particular  care 
be  taken  to  remove  from  the  steam  before  it  enters  the  turbine,  all  moist- 
ure and  foreign  matter.  Wet  steam  will  do  no  harm  of  itself,  but  has 
a  bad  effect  on  the  economy  of  the  turbine  since  its  presence  acts  as  a 
brake.  The  amount  of  oil  present  in  the  exhaust  from  the  reciprocating 
engines,  has  been  found  in  several  instances  choking  the  steam  passages 
through  the  blades.  Pure,  clean  oil  itself  would  pass  through  a  turbine 
without  accumulating,  but  in  cases 
where  the  boilers  sometimes  foam, 
discharging  sulphates,  carbonates 
or  other  solid  matter  with  the 
steam,  this  mixes  with  the  oil  to 
form  a  gummy  deposit  on  the 
blades  which  is  exceedingly  difficult 
to  remove. 

Any  of  the  turbines  mentioned 
above,  whether  equipped  with  a 
governor  regulating  the  admission 
of  steam  or  not,  will  naturally 
have  a  speed -limiting  governor  or 
automatic  throttle  which  will 
prevent  the  machine  from  running 

away  Should  Other  means  Of  Control  Fig.  x.-Constant  Pressure  Valve 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES  7 

fail.  In  speaking  of  turbines  operating  without  a  governor,  the  reference 
is,  therefore,  to  a  device  for  regulating  the  machine  within  close  limits  of 
speed,  and  while,  for  reasons  which  will  be  shown  hereafter,  the  most 
economical  performance  of  the  turbine  will  generally  not  be  obtained 
when  it  is  under  such  control,  it  may  be  desirable  to  equip  the  turbine 
with  such  a  governor  so  that  it  could  be  operated  as  an  independent  unit 
in  the  case  of  a  failure  of  the  reciprocating  apparatus.  When  operating 
normally,  however,  the  governor  may  be  adjusted  by  means  of  a  speed- 
changing  screw  so  that  the  governor  valve  will  remain  wide  open  at  all 
times,  and  thus  all  the  exhaust  steam  will  be  used  by  the  turbine  and  none 
wasted  to  the  atmosphere.  In  this  event,  the  governor  would,  of  course, 
control  the  turbine  should  the  speed  rise  materially  above  the  normal. 
A  governor  might  also  be  provided,  controlling  the  admission  of  live 
steam  alone,  thus  saving  the  expense  of  a  low  pressure  valve  and  permit- 
ting proper  operation  of  the  turbine  with  live  steam  when  there  was  no 
low  pressure  steam  available,  a  condition  which  might  arise  due  to  ac- 
cident to  the  machine  from  which  the  exhaust  was  passing  to  the  turbine. 

It  will  be  understood  that  in  any  of  the  foregoing  cases,  the  applica- 
tion may  be  made  with  direct  as  well  as  alternating  current  generators, 
the  only  difference  being  that  with  direct  current  generators  the  speed 
will  rise  slightly  as  the  proportion  of  load  taken  by  the  low  pressure 
turbine  rises,  or  as  the  quantity  of  exhaust  steam  increases,  this  rise  in 
speed  being  in  accordance  with  the  voltage  characteristics  of  the  gen- 
erator. For  the  same  reason,  it  is  possible  to  vary  the  speed  of  the 
turbine  slightly  by  manual  adjustment  of  the  field  rheostat. 

Obviously  when  the  turbine  generator  is  an  alternator  and  operates 
in  multiple  with  others  driven  by  reciprocating  engines,  the  starting  of 
the  unit  is  extremely  simple.  The  process  of  synchronizing  need  not  be 
resorted  to,  as  it  is  customary,  before  starting  the  reciprocator  and  tur- 
bine, to  establish  the  field  charge.  Therefore,  when  the  reciprocator  is 
started,  the  low  pressure  turbines  will  come  up  to  speed  with  it  in  syn- 
chronism. 

Another  consideration  that  should  be  borne  in  mind  is  that  low 
pressure  turbines  may  be  installed  in  conjunction  with  engines  previously 
operated  condensing,  as  well  as  those  performing  noncondensing  service. 
The  gain  will,  of  course,  be  somewhat  less,  but  roughly  speaking,  the  use 
of  30%  more  steam  than  was  required  by  the  reciprocators  operating 
condensing,  will  produce  75%  more  power  when  these  are  converted  to 
noncondensing  operation  and  their  exhaust  passed  through  a  low  pressure 
turbine  of  proper  proportions.  Condensing  engines  so  converted,  do  not 
necessarily  suffer  reduction  of  capacity  because  of  the  back  pressure  to 
which  they  are  subjected.  On  the  contrary,  the  capacity  of  such  an  engine 
may  be  increased  if  it  is  a  high  expansion  ratio  engine;  i  e.,  providing  the 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


JJ 


II 

o    w 


-I 


II 


Il 


n 


.5    « 


gSl 

o 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES  9 

generator  will  stand  being  increased  in  ratio  and  providing  that  the  high 
pressure  cylinder  of  the  engine  has  enough  volume  to  pass  the 
required  amount  of  steam  without  taking  steam  for  the  entire  length  of 
its  stroke.  In  many  cases  it  is  well  worth  while  to  make  provision  for 
this  increased  capacity  by  such  modifications  of  the  generator  as  increased 
air  circulation,  etc.  The  reason  for  this  increase  in  capacity  is  that  in 
laying  out  the  altered  indicator  card  of  the  engine  for  a  higher  back 
pressure,  the  cut-off  in  the  high  pressure  cylinder  will  have  to  be  moved 
along  to  a  point  late  enough  so  that  expansion  in  the  low  pressure  cylinder 
will  not  result  in  looping,  or,  in  other  words,  to  a  final  pressure  at  the 
opening  of  the  low  pressure  cylinder  exhaust  valve,  lower  than  that  of 
the  line  to  the  turbine.  Under  these  conditions  it  will  be  found  that  the 
top  portion  of  the  card  will  be  increased  in  area  or  fattened,  adding  more 
area  to  the  total  than  is  taken  from  the  lower  portion  by  raising  the  back 
pressure  to  that  of  the  atmosphere  or  thereabouts. 

There  will  sometimes  be  found  compound  engines  operating  con- 
densing in  which  release  in  the  low  pressure  cylinder  takes  place  at  a 
relatively  high  pressure,  possibly  close  to  atmospheric.  If  a  low  pressure 
turbine  were  applied  in  this  case  with  a  view  of  obtaining  increased  over- 
all efficiency,  the  engine  capacity,  instead  of  being  increased,  will  be 
decreased  since  the  possibility  of  adding  area  to  the  upper  part  of  the 
card  does  not  exist. 

Having  shown  the  classes  into  which  low  pressure  turbine  install- 
ations naturally  divide,  and  the  general  considerations  which  affect  all 
such  installations,  the  remaining  pages  will  be  devoted  to  a  more  detailed 
discussion  of  the  application  of  such  machines  in  the  various  classes 
indicated.  It  will  be  well  to  recall  at  this  point  that  the  primary  func- 
tion of  a  low  pressure  turbine  installation  is  to  produce  the  maximum 
obtainable  amount  of  power  per  unit  of  steam  generated  in  the  boilers, 
and  that,  therefore,  every  care  should  be  taken  that  all  steam  so  gener- 
ated, will  be  used  in  the  most  efficient  manner  possible. 

Case  A :  This  is  the  simplest  form  of  low  pressure  turbine  install- 
ation, and  is  shown  diagramatically  in  Fig.  3.  From  the  diagram  it  will 
be  seen  that  the  turbine  and  reciprocating  engine  are  tied  together  elec- 
trically and  that  no  governor  is  indicated  on  the  turbine,  although  as 
suggested  before,  one  may  be  provided  so  arranged  that  it  is  normally 
out  of  action  but  will  control  the  turbine  when  desired.  If,  for  instance, 
the  reciprocator  should  be  disabled,  the  turbine  could  still  operate  if  so 
equipped.  If  the  turbine  drives  an  alternating  current  generator,  it 
will  take  its  share  of  the  load  in  proportion  to  the  steam  available.  As 
it  tends  to  forge  ahead  of  the  reciprocating  engine  generator,  it  will  take 
more  of  the  load,  thus  removing  load  from  the  reciprocator  and  reducing 
the  supply  of  exhaust  steam.  The  self-regulation  is,  therefore,  perfect, 


10 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


the  governor  on  the  reciprocating  engine  controlling  the  amount  of  steam 
to  the  complete  system.  In  case  the  turbine  and  reciprocator  drive 
direct  current  generators,  as  the  turbine  tends  to  take  more  load,  the 
speed  increases  slightly,  and  with  that,  the  voltage,  and  the  system  is 
again  self -regulated. 

There  is  a  great  inherent  advantage  in  this  application  of  a  low 
pressure  turbine  which  is  directly  due  to  the  variable  pressure  existing 
in  the  receiver  or  pipe  line  between  the  engine  and  turbine.  The  ab- 
solute inlet  pressure  of  a  turbine  varies  directly  with  the  amount  of  steam 
passing  through  it.  Reciprocating  engines  designed  for  either  conden- 
sing or  noncondensing  operation  generally  have  such  a  valve  setting  as 
will  normally  provide  a  considerable  pressure  at  the  point  of  release  of 
the  low  pressure  cylinder  so  that  when  the  load  on  the  engine  is  light, 
the  indicator  card  will  not  loop.  Such  an  engine,  therefore,  when 
carrying  full  load  does  not  completely  expand  the  steam  within  the 
cylinder,  and  this  is  a  serious  source  of  loss  in  engines  designed  for  op- 
eration on  variable  load.  In  this  case  when  the  low  pressure  turbine  is 
applied,  there  is  no  necessity  for  this  provision  of  pressure  at  the  point 
of  release.  The  valve  setting  may  be  made  such  that  with  normal  load, 
expansion  within  the  cylinder  will  carry  the  steam  to  approximately 
atmospheric  pressure,  or  perhaps  a  half  pound  above  this,  in  order  to 
pass  the  steam  readily  through  the  ports.  The  low  pressure  turbine 
blading  proportions  will  then  be  made  such  that  the  inlet  pressure  will 


Fig.  3. — Case  A 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES  11 

be  equal  to  atmosphere  when  passing  the  quantity  of  saturated  steam 
exhausted  by  the  engine  when  operating  at  normal  load.  If,  now,  the 
quantity  of  steam  taken  by  the  reciprocating  engine  is  reduced  say  to 
one-half,  the  absolute  inlet  pressure  of  the  turbine  will  similarly  be  one  - 
half.  The  high  pressure  cylinder  of  the  recipocator  will  have  its  cutoff 
reduced  approximately  one-half  and  the  extension  of  steam  within  the 
low  pressure  cylinder  will  continue  to  a  point  corresponding  approxi- 
mately to  the  turbine  inlet  pressure.  There  will  be  no  looping  of  the 
card  and  the  expansion  of  the  steam  through  the  cycle  will  be  effected  in 
an  ideal  manner.  It  is  hardly  necessary  to  point  out  that  a  reciprocating 
engine  designed  to  expand  the  steam  to  atmospheric  pressure  at  full 
load  would  expand  it  to  a  point  below  this  pressure  at  light  loads,  and 
if  the  back  pressure  were  maintained  constant,  would  soon  slam  the 
valves  to  pieces. 

It  should  be  noted  that  the  proportion  of  load  carried  by  the  engine 
and  the  turbine  respectively,  varies  with  the  quantity  of  steam.  The 
greater  the  total  load,  the  greater  the  proportion  taken  by  the  turbine, 
due  of  course,  to  the  variable  receiver  pressure.  The  higher  this  becomes, 
the  greater  is  the  heat  range  through  which  the  turbine  works.  This 
particular  feature  is  of  such  an  extensive  nature  that  a  complete  dis- 
cussion of  it  is  impossible  in  the  space  here  available. 

In  turbine  applications  of  this  kind,  as  several  of  those  that  follow, 
a  synchronous  or  induction  type  generators  may  be  employed.  These 
have  the  advantage  of  requiring  no  field  adjustment. 

Case  B:  This,  as  stated  previously,  is  similar  to  Case  A,  except 
that  the  larger  the  number  of  units  involved,  the  greater  is  the  number 
of  combinations  possible.  A  plant  of  this  type  is  shown  in  Fig.  2.  The 
desirable  arrangement  in  installing  a  number  of  low  pressure  turbines 
is  to  equip  each  with  its  own  condenser.  This  conduces  to  flexibility 
of  the  plant  over  that  obtainable  where  one  large  condenser  is  installed 
to  serve  all  the  turbines  as  it  materially  reduces  the  probability  of  a 
shut-down  due  to  condenser  failure.  In  plants  of  any  considerable  size, 
the  desirable  method  of  applying  low  pressure  turbines  is  to  install  one 
for  each  engine,  the  plant  becoming  a  multiple  of  Case  A  and  each 
engine-turbine  unit  being  comparable  to  a  simple  engine  which  has  been 
compounded  by  the  addition  of  a  low  pressure  cylinder,  the  only  difference 
being  that  the  two  portions  are  tied  electrically  instead  of  mechanically. 
The  regulation  of  the  system  is  exactly  the  same  as  that  of  Case  A, 
except  that  the  capacity  of  the  low  pressure  turbine  may  be  too  small  to 
handle  the  amount  of  steam  exhausted  from  the  number  of  engines  that 
are  in  operation.  Part  of  the  exhaust  steam  would,  therefore,  escape  to 
the  atmosphere  and  be  wasted.  A  low  pressure  turbine  is  generally 
designed  so  that  the  maximum  load  on  the  generator  will  be  reached 


12 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


Fig.  4  &  5 — Case  C  plant  of  an  Ohio  steel  company  in  which  there  are  two  600  kw.  Low 
Pressure  Turbines  tied  through  rotary  converters  to  direct  current  engine-driven 
alternators.  Since  some  exhaust  steam  is  available  from  other  sources,  governors  are 
fitted  to  permit  operation  when  the  engine  units  are  shut  down. 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES  13 

when  the  inlet  pressure  on  the  turbine  is  about  20  or  21  pounds  per  sq.  in. 
absolute,  and  it  is,  therefore,  customary  to  set  the  relief  valve  on  the  low 
pressure  pipe  line  to  open  at  about  this  pressure. 

The  same  consideration  affects  the  variable  back  pressure  on  the 
engines  in  this  case  as  in  Case  A. 

In  installations  of  these  two  classes  care  should  be  exercised  that  no 
air  may  leak  into  the  system  when  the  receiver  pressure  is  below  that  of 
the  atmosphere,  since  such  air  will  seriously  affect  the  performance  of 
the  condenser,  the  vacuum  will  be  reduced,  and  the  full  possibility  of 
gain  from  the  use  of  the  turbines  will  not  be  realized.  Obviously,  the 
greater  the  number  of  engines  and  the  longer  or  larger  the  exhaust  pipe 
system,  the  greater  attention  must  be  paid  to  this  point.  It  is  frequently 
necessary  to  provide  special  double  packing  rings  on  the  rods  of  all  the 
engines,  such  packing  being  so  arranged  that  steam  at  a  pressure  slightly 
greater  than  the  atmosphere  is  admitted  between  the  two  rings  and  any 
leak  into  steam  spaces  will,  therefore,  be  of  steam  rather  than  of  air. 

In  some  case  A  and  B  plants  there  are  found  additional  reciprocating 
engines  and  other  steam  using  machines  carrying  loads  having  no  possible 
relation  to  the  electrical  load.  Air  compressors  and  pumps  would  be 
examples.  The  steam  from  such  sources  is,  however,  generally  insuf- 
ficient to  drive  the  turbine  except  during  some  abnormal  condition  of 
infrequent  occurrence,  such  as  a  small  electrical  load  occurring  at  the 
same  time  with  a  high  pump  and  compressor  load.  Where  this  factor  is 
involved,  it  would  be  advisable  to  equip  the  turbine  with  a  governor, 
setting  the  speed  adjusting  screw  so  that  the  turbine  would  regulate  at 
about  6%  above  normal  speed.  Operation  under  normal  conditions, 
therefore,  would  take  place  as  if  no  governor  were  applied,  but  as  soon  as 
the  over-supply  of  steam  occurred,  the  governor  would  exercise  perfect 
control  over  the  turbine.  In  making  this  provision,  it  would,  of  course, 
be  necessary  to  keep  the  live  steam  valve  to  the  turbine  closed,  except 
when  it  was  desired  to  operate  on  steam  direct  from  the  boilers. 

In  the  operation  of  such  a  plant,  provision  should  be  made  for  turn- 
ing over  each  additional  engine  which  is  put  on  to  load  for  a  few  revolu- 
tions, exhausting  to  the  atmosphere,  in  order  to  completely  expel  from 
it  the  air  which  has  collected  in  the  cylinders  and  receiver  while  it  was 
at  rest.  Otherwise  this  air  discharged  into  the  exhaust  system  and  pass- 
ing to  the  condenser  is  very  apt  to  cause  a  loss  of  vacuum,  and  conse- 
quently to  interfere  with  the  operation  of  the  entire  station. 

Case  C :  In  the  two  classes  of  installation  just  discussed  the  gen- 
erators of  the  low  pressure  turbines  and  reciprocating  engines  were  all 
of  the  same  electrical  characteristics.  In  Case  C,  the  generators  driven 
by  the  reciprocating  apparatus  may  be  either  direct  or  alternating 
current  and  the  generators  driven  by  the  low  pressure  turbine  different 


14 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


from  all  or  some  of  them.  An  electric  railway  power  house  is  often  of 
this  type,  the  direct  current  machines  supplying  the  local  feeders  and 
the  alternating  current  derived  from  existing  engines  or  to  be  derived 
from  the  turbine  distributing  to  distant  substations.  If  the  low  pressure 
turbine  in  this  plant  were  equipped  with  a  governor  arranged  to  meet 
only  the  demands  for  alternating  current,  steam  would  be  wasted  when 
the  A.  C.  load  became  light  and  the  installation  would  be  unsatisfactory 
for  that  reason.  There  is  a  further  objection  that  to  meet  the  demands 
of  certain  alternating  current  loads  occurring  at  times  when  the  direct 
current  load  was  light,  it  might  be  necessary  to  install  some  high  pressure 
alternating  current  apparatus  or  to  furnish  the  low  pressure  turbine  with 
live  steam  which  would  again  be  uneconomical  operation.  A  convenient 


Separating 


A 

C.  Bus-Bars 

n 

i 

? 

_ 

j_ 

rv 

^j 

^XnJ 
(I   j 

& 

UH 

J 

J°l 

Low  Pressure  Turbine 

r 

, 

Converter 


Fig.  6.— Case  C 

and  satisfactory  solution  is  afforded  by  tying  the  alternating  and  direct 
current  systems  together  by  means  of  a  rotary  converter  or  motor 
generator  set,  as  shown  diagrammatically  in  Fig.  6.  This  will  insure 
the  use  of  all  the  steam  in  the  most  economical  manner  possible.  Any 
lack  of  balance  between  the  electrical  loads  will  be  taken  care  of  by  the 
rotary,  the  load  on  the  turbine  and  engines  being  divided  for  best  economy 
irrespective  of  the  division  in  the  total  output  of  the  station.  Such  a 
plant  might  possibly  be  undesirable  where  lighting  service  is  involved 
due  to  the  voltage  variation  of  the  rotary  in  swinging  over  from  full  load 
alternating  direct  current  operation  to  full  load  direct  alternating  current 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


15 


operation,  amounting  probably  to  10%.  In  the  majority  of  cases,  how- 
ever the  direct  current  engine  load  will  predominate  over  the  alternating 
current  demand,  and  the  rotary  or  M.  G.  set  will  be  uni-directional,  in 
which  event  lighting  could  be  satisfactorily  served.  Even  in  the  pre- 
viously mentioned  case  the  use  of  separate  excitation  in  conjunction  with 
a  Tirrel  regulator  for  the  M.  G.  set  will  remedy  the  difficulty  of  voltage 
variation. 

Another  method  of  solving  the  same  general  problem  is  to  connect 
with  the  reciprocating  engines,  directly  or  by  belt,  a  synchronous  A.  C. 
generator  in  multiple  with  that  driven  by  the  low  pressure  turbine. 
This  synchronous  motor  must  have  sufficient  capacity  for  the  maximum 
unbalancing  of  the  direct  and  alternating  current  loads. 

By  making  use  of  this  expedient  the  reciprocator  automatically 
takes  sufficient  steam  for  the  needs  of  the  whole  system  with  maximum 
economy  and  quite  ideal  regulation,  at  the  same  time  satisfying  the 
primary  requirement  that  under  no  condition  of  operation  shall  steam 
escape  to  atmosphere. 

Case  D :  Many  plants  contain  a  large  number  of  engines,  pumps  or 
other  steam-using  prime  movers  which  do  not  drive  electrical  apparatus. 
The  exhaust  from  these  may  be  collected  and  brought  to  a  common 
point  at  which  it  is  used  in  a  low  pressure  turbine  which  must  obviously 
be  controlled  by  a  governor.  In  such  installations  when  the  demand  of 
the  turbine  for  steam  is  greater  than  the  amount  supplied  by  the  various 
sources,  the  pressure  in  the  piping  system  will  fall  until  it  becomes  less 
than  that  of  the  atmosphere,  and  under  this  condition  the  inleakage  of 
air  is  practically  unavoidable.  It  is,  therefore,  essential  to  place  a  con- 
stant pressure  valve  in  the  low  pressure  steam  line  adjacent  to  the  turbine, 
thus  insuring  the  maintenance  of  a  pressure  in  this  line  slightly  greater 
than  that  of  the  atmosphere,  at  such  times  as  the  available  amount  of 


Fig.  7. — Case  D 


16 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


:         g< 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES  17 

exhaust  steam  is  not  sufficient  to  so  maintian  it  if  the  turbine  were 
drawing  from  this  line.  A  considerable  saving  can  generally  be  effected 
by  this  expedient  since  it  makes  unnecessary  a  large  investment  for  a 
heavy  pipe  line  to  prevent  such  inleakage.  A  constant  pressure  valve  is 
shown  Fig.  1.  A  relief  valve  should  also,  of  course,  be  fitted  to  the  line 
so  that  any  steam  not  used  by  the  turbine  will  escape  to  the  atmosphere. 
As  the  service  in  this  case  may  be  such  that  the  supply  of  exhaust  steam 
is  not  always  sufficient  to  carry  the  load  imposed  on  the  turbine,  an 
auxiliary  valve  under  the  control  of  the  turbine  governor  should  be 
provided  to  admit  live  steam  to  the  turbine  when  necessary. 

At  first  glance  the  use  of  live  steam  in  the  turbine  designed  for  low 
pressure  work  would  appear  to  be  poor  engineering,  but  in  general  this 
is  not  the  case.  The  fact  that  the  supply  of  low  pressure  steam  is  in- 
sufficient generally  indicates  that  some  of  the  reciprocating  apparatus 
has  been  relieved  of  its  load,  and  since  the  boiler  plant  is  producing  steam 
at  a  rate  sufficient  to  carry  the  former  load  on  the  reciprocators,  the 
safety  valves  will  soon  blow  unless  it  is  used  in  some  way  or  other.  Its 
admission  to  the  turbine  will,  of  course,  prevent  this  condition  from 
arising,  unless  the  turbine  load  also  is  considerably  reduced.  It  is, 
therefore,  quite  frequent  practice  to  pipe  the  safety  valves  on  the  boilers 
into  the  low  pressure  system  between  the  engines  and  the  turbine.  Con- 
ditions such  as  these  suggested  immediately  above  are  most  frequently 
found  in  rolling  mills  or  hoisting  sheds  where  the  reciprocators  are  in 
intermittent  service.  If,  however,  there  is  a  larger  proportion  of  the 
time  when  it  is  desired  to  operate  such  a  turbine  on  high  pressure  steam, 
the  correct  solution  is  that  of  Case  H.  Fig.  7  shows  a  plant  of  this  type 
diagrammatically. 

Case  E:  There  are  a  good  many  plants  where  the  engine  and 
turbine  loads  are  independent  or  cannot  be  yoked  together.  Thus  the 
load  on  the  turbines  may  be  very  much  lighter  than  that  on  the  recip- 
rocating engine,  and  if  the  installation  were  made  as  in  Case  D,  the  ex- 
cess of  steam  from  the  reciprocating  engines  would  escape  to  the  atmos- 
phere and  be  wasted.  Case  E,  therefore,  makes  provision  for  operating 
a  reciprocating  engine  partially  condensing  under  such  conditions.  The 
turbine  governor  controls  a  valve  which,  instead  of  admitting  steam  to 
the  turbine,  allows  the  exhaust  from  the  reciprocator  to  pass  to  the  con- 
denser. If  there  is  no  load  at  all  on  the  turbine,  this  valve  would  stay 
wide  open  and  the  engine  would  operate  with  substantially  full  vaccum. 
As  the  turbine  load  increases,  the  governor  tends  to  close  this  valve, 
making  a  sufficient  difference  between  the  inlet  and  exhaust  pressures  of 
the  turbine  so  that  the  desired  load  may  be  carried.  Finally  the  governor 
valve  may  be  entirely  closed,  thus  forcing  all  the  steam  from  the  recip- 
rocator to  pass  through  the  turbine. 


18 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


8I 

.2  o 

i 


II 


1! 

ta 


32 

r 

o 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


19 


Obviously  the  advantage  of  this  arrangement  is  that  whenever  the 
load  on  the  turbine  falls  below  normal,  the  reciprocating  engine  will 
obtain  the  benefit  of  some  vacuum.  The  turbine  inlet  is,  of  course, 
provided  with  a  butterfly  valve  connected  to  the  governor  so  that  the 
turbine  will  be  prevented  from  running  away  if  the  load  be  entirely 
removed,  or  if  for  some  external  cause  the  reciprocators  disgorge  an  inor- 
dinate amount  of  steam  which  could  not  get  through  the  bypass  without 
a  material  rise  of  pressure  in  the  exhaust  steam  line.  In  some  cases  the 
installation  described  might  be  better  arranged  by  treating  it  as  Case  C, 
using  a  synchronous  generator  tie. 

Case  F:  This  is  a  modification  of  Case  C  installation  and  has 
found  a  wide  application  among  the  cotton  mills  of  New  England.  It  is 
again  a  case  where  the  low  pressure  turbine  and  reciprocating  engine 
loads  are  entirely  different,  but  a  tie  may  be  established  between  them  and 
in  this  way  all  the  exhaust  steam  used.  As  in  Case  A  the  turbine 
normally  operates  independently  of  a  governor,  although  generally  one 


Separating  Receiver  Governor.ControUed 

By-Pass  Valve 


Belt  Pulley 


Fig    10. — Case  E 


20 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


H 

>> 
a 
S.S 


w  S 


5  2 

''rt  a 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


21 


would  be  fitted  to  provide  for  operation  should  the  reciprocating  engine 
go  out  of  service.  An  ideal  case  of  such  an  installation  is  found  where  the 
reciprocating  engine  is  driving  a  mill  through  line  shafting  and  it  is 
desired  to  enlarge  the  mill  using  motor  drive,  for  individual  machines,  or 
to  increase  the  power  plant  capacity  for  any  purpose.  The  low  pressure 
turbine  is  installed  and  electrically  connected  to  a  synchronous  generator 
belted,  or  otherwise  connected,  to  the  mill  shafting.  This  synchronous 
generator  will  either  take  energy  from,  or  give  energy  to  the  mill  shafting 


Fig.  12 — Case  F 

in  accordance  with  the  varying  demands  of  the  power  and  electrical 
systems.  Should  the  mill  load  increase,  the  governor  on  the  reciprocat- 
ing engine  will  admit  more  steam  which  will  pass  through  to  the  turbine. 
The  excess  load  produced  by  the  turbine  is  delivered  to  the  mill  shafting 
through  the  synchronous  generator.  An  increase  of  the  motor  load 
driven  by  the  low  pressure  turbine,  means  that  it  will  tend  to  slow  down 
and  energy  will  be  taken  from  the  mill  shafting  by  the  synchronous  gener- 
ator whereupon  the  reciprocating  engine  governor  will  admit  more  steam 
to  the  engine.  If  all  the  energy  from  the  low  pressure  turbine  is  to  be 
employed  in  driving  the  mill  and  the  purchaser  does  not  desire  to  operate 


22 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


s 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


23 


motors  or  lighting  circuits,  the  turbine  may  be  connected  to  the  mill 
shaft  directly  through  gears  and  all  the  advantages  of  the  Case  A  install- 
ation may  be  realized,  the  connection  being  mechanical  instead  of  elec- 
trical. 

Case  G :  Where  the  supply  of  the  exhaust  steam  is  obtained  from 
rolling  mill  or  hoisting  engines  and  is  therefore,  very  variable,  there  may 
often  arise  a  condition  where  there  is  a  complete  cessation  of  the  flow  of 
exhaust  steam.  To  continue  the  operation  of  the  turbine,  it  will, 
therefore,  be  necessary  to  supply  live  steam  or  else  employ  a  heat  storage 


Fig.  14— Ca  se  G 

means,  such  as  a  regenerator,  of  sufficient  capacity  to  bridge  over  the 
period  when  no  exhaust  is  being  furnished  by  the  engines.  This  case 
will  be  recognized  as  an  elaboration  of  Case  D,  differing  only  in  that 
means  are  provided  for  storing  heat  when  an  excessive  exhaust  steam 
supply  is  available  and  giving  up  this  stored  heat  when  there  is  an 
insufficient  supply.  The  regenerator  consists  generally  of  a  large  vessel 
partially  filled  with  water  through  which  the  exhaust  steam  passes  on 
going  from  the  reciprocating  engine,  the  water  being  thus  maintained  at 
the  temperature  of  the  steam.  It  is  usual  to  design  a  regenerator  to 
operate  between  the  limits  of  atmospheric  pressure  and  5  or  10  Ibs. 
gauge,  the  lower  limit  being  selected  in  order  to  avoid  the  possibility  of 


24 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


O   bO 


<i>  tn  •  j 

O         41 

ll: 


1! 


1^5 

-0    W>3° 

«! 


S 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES  25 

inleakage  of  air.     To  indicate  the  action  of  a  regenerator,  the  following 
actual  case  may  be  cited: 

Assuming  that  the  pressure  limits  for  its  operation  are  14.7  Ibs.  and 
25  Ibs.  absolute;  i.  e.,  atmospheric  pressure  and  10  Ibs.  gauge,  the  cor- 
responding temperatures  will  be  212°  and  240°  Fahrenheit,  the  difference 
between  which  is  28°.  Assume  that  the  water  in  the  regenerator  is  at 
212°  and  the  steam  at  a  corresponding  pressure,  and  that  the  steam 
delivered  by  the  reciprocating  engine  is  exactly  equal  to  the  demand  of 
the  low  pressure  turbine.  So  long  as  this  condition  exists,  the  steam 
flows  through  the  water  in  the  regenerator  and  no  heat  is  taken  up  or 
given  out.  Now  assume  the  load  on  the  reciprocator,  and  consequently 
the  amount  of  steam  it  discharges,  to  increase  while  the  demand  of  the 
turbine  for  steam  remains  constant.  Then  the  pressure  within  the 
regenerator,  and  accordingly  the  temperature  of  the  steam,  will  begin  to 
rise,  resulting  in  an  increase  in  the  temperature  of  the  water  if  provision 
is  made  for  intimately  mixing  the  steam  with  it.  As  the  pressure  in- 
creases, the  steam  comes  in  at  a  temperature  higher  than  the  water  and 
is  so  in  part  condensed,  which  assists  in  maintaining  the  temperature 
of  the  water  corresponding  to  that  of  the  steam.  This  process  may 
continue  until  the  pressure  within  the  regenerator  reaches  25  Ibs.  at 
which  point  the  relief  valve  has  been  set.  Under  these  conditions,  each 
pound  of  water  within  the  regenerator  will  have  absorbed  28.3  heat  units. 
Now  suppose  the  flow  of  exhaust  steam  from  the  reciprocating  engine  to 
absolutely  cease,  in  which  event  the  pressure  within  the  regenerator 
begins  to  fall  because  the  turbine  continues  to  draw  steam  from  it.  When 
this  pressure  has  reached  14.7  Ibs.,  the  water  in  the  regenerator  will  have 
evaporated  until  its  temperature  corresponds  to  that  of  steam  at  this 
pressure;  i.  e.,  212°,  and  each  pound  of  water  in  the  regenerator  will  have 
given  out  28.3  heat  units,  exactly  equivalent  to  the  amount  absorbed 
while  the  pressure  was  rising.  The  mean  latent  heat  of  evaporation 
through  this  range  being  961  units,  each  pound  of  water  so  reduced  in 
temperature  will  liberate  .0294  Ibs.  of  steam.  Obviously,  in  installing  a 
regenerator,  the  important  point  to  determine  is  the  length  of  time  the 
regenerator  will  be  expected  to  carry  full  load  on  the  turbine  without  any 
supply  of  steam  from  the  engines.  From  the  above  figures  it  will  be  seen 
that  if  this  is  long,  the  quantity  of  water  within  the  regenerator  must  be 
very  great.  Then  again,  the  pressure  limits  assumed  in  the  above 
example  were  quite  wide.  In  a  good  many  installations  it  would  not  be 
possible  to  impose  10  Ibs.  back  pressure  on  the  reciprocating  engines. 
Reference  to  this  matter  is  made  because  generally  the  disposition  of  a 
prospective  installer  of  a  regenerator  is  to  make  the  time  element  much 
greater  than  is  necessary.  Some  early  installations  of  regenerators 
provide  for  six  to  seven  minutes  of  operation  without  a  supply  of  exhaust 


26 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


I 
I 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES  27 

steam,  whereas  six  or  seven  seconds  would  have  been  better.  For  in- 
stance, in  steel  mill  practice,  if  the  exhaust  from  a  blooming  mill  engine 
is  to  be  considered,  the  time  element  should  bear  reference  to  the  period 
between  the  passes  of  an  ingot,  and  to  the  maximum  time  from  the  last 
pass  of  one  ingot  to  the  first  pass  of  a  new  ingot,  and  the  regenerator 
should  not  be  designed  to  cover  such  delays  as  would  arise  due  to  the 
clogging  of  the  mills  or  because  a  new  ingot  might  not  be  ready  to  be 
bloomed. 

As  was  indicated  in  Case  D,  there  is  another  source  of  exhaust  steam 
available  when  such  an  unforseen  interruption  arises.  The  boiler  plant 
is  being  continually  fired,  and  if  the  demand  for  steam  is  interrupted  in 
this  unusual  way,  the  safety  valve  will  blow.  The  steam  escaping  in 
this  way  should  be  led  direct  to  the  regenerator  so  that  the  turbine  will  con- 
tinue to  carry  its  load  without  drawing  live  steam.  The  best  method  of 
providing  for  this  is  a  cross-connection  between  the  steam  and  exhaust 
lines  of  the  engine  in  which  is  placed  a  spring-loaded  valve  set  to  blow  at 
a  few  pounds  lower  than  the  safety  valves. 

There  will  always  be  periods  of  more  or  less  duration  when  the  re- 
generator cannot  furnish  enough  steam  for  the  turbine  and  the  only 
alternative  is  to  supply  live  steam.  This  has  been  frequently  accom- 
plished by  admitting  steam  to  the  regenerator  through  a  reducing  valve 
so  set  that  when  the  pressure  within  the  regenerator  reaches  the  lower 
limit,  steam  will  be  admitted.  Such  valves  are,  however,  generally 
unsatisfactory  and  troublesome,  and  it  is  better  practice  to  employ  for 
this  purpose  the  secondary  or  live  steam  valve  under  control  of  the  turbine 
governor.  This  valve  may  be  arranged  to  admit  steam  either  to  the  inlet 
side  or  outlet  side  of  the  regenerator.  If  admitted  to  the  inlet  side,  no 
water  hammers  will  be  produced,  a  condition  which  might  arise  with 
admission  on  the  outlet  side  if  the  regenerators  were  arranged  for  wide 
pressure  limits.  The  disadvantage  of  admission  on  the  inlet  side,  how- 
ever, is  that  the  pressure  in  the  regenerator  cannot  be  raised  without 
condensing  steam  therein,  thus  necessitating  supplying  a  large  amount  of 
live  steam  to  tide  over  temporary  heavy  loads  on  the  turbine.  In  such 
an  arrangement  it  is  conceivable  that  when  the  exhaust  steam  again  flows 
into  the  regenerator,  the  latter  being  already  partially  charged,  it  will  be 
unable  to  absorb  all  of  the  supply  and  some  of  it  will  pass  through  the 
relief  valve  to  the  atmosphere,  thus  violating  the  primary  consideration 
for  which  the  regenerator  was  installed.  If  live  steam  is  to  be  admitted, 
therefore,  and  the  regenerator  is  arranged  to  work  within  wide  limits, 
arrangement  should  be  made  to  admit  it  between  the  turbine  and  re- 
generator. It  may  prove  desirable  to  install  a  check  valve  between  the 
point  in  the  pipe  where  the  live  steam  is  admitted  and  the  regenerator  so 
that  if  the  regenerator  were  at  the  lower  pressure  limit  and  no  steam  was 


28 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


f 

II 


§1 

II 


THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 


29 


being  admitted  to  it  and  the  load  on  the  turbine  was  such  that  more 
steam  was  required,  the  pressure  in  the  regenerator  would  not  be  raised 
without  the  temperature  of  the  water  rising  correspondingly.  Obviously, 
if  the  pressure  in  the  regenerator  were  raised  by  admitting  steam  in  the 
reverse  direction;  that  is,  without  passage  through  the  water  from  the 
secondary  valve  on  the  turbine,  then  when  steam  comes  from  the  re- 
ciprocating engines  in  the  usual  direction,  it  would  be  at  a  temperature 
higher  than  that  of  the  water  within  the  regenerator  and  might  cause 
hammering,  the  violence  of  which  would  correspond  to  the  temperature 
difference. 

Case  H :  Occasionally  the  conditions  in  a  plant  are  such  that  it  is 
desirable  to  utilize  a  quantity  of  exhaust  steam  which  is  intermittently 
available,  but  there  are  also  long  periods  of  time  unrelated  to  this  supply 


Fig.  18.— Case  H. 

over  which  the  output  of  the  turbine  is  desired  and  when  it  would, 
therefore,  be  preferable  to  arrange  to  operate  on  live  steam.  Or  again 
there  may  be  constantly  available  an  amount  of  exhaust  steam  insuf- 
ficient to  carry  the  average  load  on  the  turbine.  The  simple  low  pressure 
turbine  does  not  have  a  high  efficiency  when  operating  with  live  steam, 
and  in  this  case,  therefore,  a  special  type  of  machine  is  employed.  This 
is  commonly  called  the  mixed  pressure  turbine;  but  as  used  here,  this 
expression  refers  specifically  to  a  turbine  having  a  separate  high  and  low 
pressure  element  in  one  cylinder.  All  such  turbines  of  course,  are  fitted 


30  THE  APPLICATION  OF  LOW  PRESSURE  TURBINES 

with  governors  and  special  valves  arranged  to  supply  high  pressure  steam 
to  appropriate  nozzles  when  the  supply  of  exhaust  steam  is  insufficient 
or  fails  entirely.  The  steam  is  then  expanded  in  the  high  pressure 
nozzles  and  its  expansive  energy  absorbed  down  to  the  pressure  at  the 
inlet  to  the  low  pressure  elements.  Such  a  turbine  is  obviously  the  same 
as  any  other  low  pressure  turbine  fitted  with  a  governor  and  live  steam 
valve,  except  that  the  live  steam  passes  through  the  high  pressure 
element  before  entering  the  low  pressure  section. 

A  large  number  of  applications  of  such  a  mixed  pressure  turbine  may 
very  conveniently  be  made  where  the  exhaust  steam  available  comes 
from  pumps  or  other  apparatus  of  that  kind,  but  it  is  generally  not  suited 
for  work  in  large  sizes,  the  most  economical  solution  of  the  problem  being 
afforded  by  the  use  of  a  small  low  pressure  turbine  and  another  complete 
expansion  machine  to  give  the  necessary  capacity.  Again  a  solution  of 
the  problem  may  be  found  in  Case  C  or  F.  It  should  be  remembered 
that  the  high  pressure  element  revolving  without  doing  work  when  the 
turbine  is  running  on  exhaust  steam,  is  a  source  of  loss  amounting  to  at 
least  2%;  and  further  than  this,  it  adds  a  complication  to  what  has  been 
previously  pointed  out  as  an  inherently  simple  machine.  In  cases  where 
they  are  desirable,  the  construction  consists  of  an  impulse  element  in 
which  the  energy  of  expansion  to  approximately  atmospheric  pressure  is 
absorbed,  the  remainder  of  the  energy  being  taken  out  through  reaction 
low  pressure  elements.  A  sectional  view  of  such  a  machine  is  shown  in 
Fig.  14.  The  control  valves  are  so  arranged  that  no  high  pressure  steam 
is  admitted  until  the  low  pressure  valve  is  fully  open,  or  in  other  words, 
until  the  turbine  is  unable  to  carry  its  load  with  the  available  supply  of 
exhaust  steam.  There  may  occasionally  arise  a  case  where  a  mixed 
pressure  turbine  could  be  used,  as  in  Case  G,  in  which  event  the  check 
valve  indicated  as  desirable  should  not  be  omitted. 


The  Westinghouse  Machine  Company 

Designers  and   Builders  of 

Steam  Turbines  Stokers 

Steam  Engines  Gas  Producers 

Gas  Engines  Pumps 

Condensers  Blowers 

Turbo  Compressors 


SALES    OFFICES 

New  York 165  Broadway 

Chicago. 39  South  La  Salle  Street 

Pittsburgh Westinghouse  Building 

Philadelphia 1003  North  American  Building 

Boston , 201  Devonshire  Street 

Atlanta : Candler  Building 

Detroit Dime  Savings  Bank  Building 

Cleveland 1117  Swetland  Building 

Indianapolis Traction  Terminal  Building 

San  Francisco Hunt  Mirk  &  Co.,  141  Second  St. 

City  of  Mexico Cia  Ingeniera,  Importadora  y  Contratista,  S.  A. 

Havana,  Cuba Galban  &  Company 

San  Juan,  Porto  Rico Porto  Rico  Construction  Co. 

Iquique,  Chile J.  K.  Robinson  &  Co. 

Tokio,  Japan Takata  &  Company 

Caracas,  Venezuela H.I.  Skilton 


GENERAL  OFFICES  AND  WORKS 

EAST  PITTSBURGH,  PA. 


^26304 


Wt 


UNIVERSITY  OF  CALIFORNIA  UBRARY 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


